Artists and film-makers use 3D computer graphics software to create 3D computer-generated images and animations. Animators use multiple computer programs, including 3D editors and non-linear editors. 3D editors (including 3D modeling, animation, layout, and rendering programs, and the like) allow a user to create and manipulate 3D models—mathematical representations of 3D objects. The user combines 3D models to create a 3D scene 101 (FIG. 1A) that includes an environment, cameras 103, lighting, and props 107. The environment is the setting of a scene, for example, an office, a house or a park. The cameras 103 are virtual cameras—computer simulated cameras that define a perspective or point-of-view to render a 3D scene from. The props 107 are 3D models of objects that are in the scene for a particular shot, for example, people and cars. Some 3D editors allow a user to create an ordered sequence of rendered 2D images of a 3D scene, commonly called a clip or a take.
Once a user has created one or more clips, a non-linear editor allows the user to arrange and modify the clips along a timeline. A user can combine multiple clips to create a time-ordered sequence of clips, commonly called a sequence. The editor is non-linear in the sense that it can randomly access frames or clips within the sequence. Non-linear editors can include other tools, for example, tools to apply video effects such as fading in or out.
Some animators begin animation by creating 3D scenes and shooting (rendering from the point-of-view of a virtual camera) multiple clips of those scenes. Then, when the animator is finished manipulating 3D objects and shooting clips, the clips are combined in a non-linear editor to create sequences and ultimately a finished animation. This creation process is familiar to traditional film directors, who shoot scenes in real life with real cameras and then combine the footage in a non-linear editing environment after all of the filming is done. Sometimes it desirable to modify a 3D scene and re-shoot a clip. However, if the 3D scene has been altered since the time when the clip was shot, it may be difficult or impossible to recreate the scene.